Production of boron by fused salt bath electrolysis



PRODUCTION OF BORON BY FUSED SALT BATH ELECTROLYSIS Hugh S. Cooper, Shaker Heights, and James C. Schaefer, Cleveland, Ohio, assignors to Walter M. Weil, Shaker Heights, Ohio No Drawing. Application January 2, 1958 Serial No. 706,582

Claims. (Cl. 204-60) This invention relates to the production of boron by fused salt bath electrolysis of potassium chloride and potassium fluoborate, and more particularly to improvements involving the production and the regeneration or rejuvenation of fused salt baths used in the production of elemental boron by electrolysis.

The production of elemental boron by the electrolysis of fused baths of potassium chloride and potassium fluo' borate is the subject of Cooper Patent No. 2,572,248, and this process has been successfully used on a commercial scale for a number of years. In the process, potassium chloride and potassium fluoborate are disassociated to form elemental boron, potassium fluoride, and chlorine gas. The electrolysis reaction may be represented by the following equation:

As the concentration of potassium fluoride in the bath increases, there is a corresponding decrease in the current carrying capacity of the bath. Continuation of electrolysis, with the attendant formation of additional potassium fluoride, eventually creates an anode-effect which reduces the current carrying capacity ofthe bath below i that at which electrolysis may be economically continued. The partially spent bath, still containing several percent -of boron as the fluoborate salt, is currently discarded in commercial operations, and a new salt mixture is introduced into-the cell.

Attempts have been made to reduce the anode-effect by maintaining a chloride concentration in the bath greater than the fluoride concentration. This can be done by introducing additional potassium chloride into the bath from time to time during electrolysis. However, the addition of substantial quantities of potassium chloride while the potassium fluoborate is being depleted by electrolysis reduces the proportion of boron salt in the bath, and this, in turn, reduces the amount of boron 2 of the bath does not decrease significantly during e1ectrolysis.

In accordance with the present invention, the regeneration or rejuvenation of fused salt baths in the production of boron by the Cooper fused salt electrolytic method is accomplished by reacting the potassium fluoride produced in the bath with a boron halide such as boron trichloride or boron fluoride, the former being preferred.

Boron trichloride reacts with potassium fluoride to form potassium fluoborate and potassium chloride as represented by the following equationr Not only does the reaction of boron trichloride with the potassium fluoride in the spent bath result in the formation ofthe two starting materials, potassium fiuoborate and potassium chloride, but these starting materials are formed in the ratio of two moles of the potassium fluoborate to 'six moles of the potassium chloride, the exact ratio in which these salts enter into the electrolytic reaction and are consumed in the process.

which canbe deposited on the cathode per hour by reducing the current that may be passed through the bath.

Thus, the method employed heretofore has a number of inherent disadvantages. For example, being a batch method, it requires frequent interruption of the electrolysis for discarding and replacing partially spent baths in order to maintain a sufficiently high cell current. Also, since no economically feasible process for recovering and reusing the components of partially spent baths has yet been proposed, the valuable components thereof are presently being discarded with a consequent economic loss.

An object of the present invention is to provide a method by which the residual salt bath components of the above-described Cooper process can be treated to regenerate the original bath components therefrom.

A further object is to provide a method for regenerating the salt bath," continuously or intermittently, in situ in the electrolytic cell in which it is being depleted.

A further object is to provide a substantially continuous fused salt electrolytic process for making boron, and particularly one in which the current carrying capacity Electrolysis of the potassium fluoborate-potassium chloride bath can be performed with a minimum of interruption by the continuous addition of boron trichloride gas to the bath while it is undergoing electrolysis. The combined electrolysis and bath regenerationreactions may be represented bythe following equation: a (3) 2BCl +KBF +KCl- 2B+3Cl +KBF +KCl In the continuous electrolytic process, the current car rying capacity of the bath can be maintained substantially at the high value prevailing in a freshly prepared bath. This substantially constant amperage can be achieved by introducing the boron trichloride gas into the bath at a rate such that it will combine with the potassium fluoridesubstantially as rapidly as itis being formed by electrolysis. In this manner, theinfluence of anode-effect is minimized. Also, maintenance of the high amperage results in the production of more elemental boronpe r unit of time than produced by the earlier process.

Although continuous regeneration of the salt baths according to the procedure described above is the preferred mode of operation, batch-wise regeneration of the bath can be accomplished, if desired, by the intermittent addition of boron trichloride at any stage of par: tial or complete exhaustion of the potassium fluoborate in the bath. The starting or initial chloride electrolytic bath also maybe prepared in accordance with the method of the present invention by beginning with fresh molten potassium fluoride and reacting itwith boron trichloride to form potassium fluoborate and potassium chloride.

Where boron trifluoride is the boron halide employed in the bath regeneration, the reaction thereof with potassium fluoride in the bath will result in the formation of potassium fluoborate only as represented by the following equation:

Thus, to complete the regeneration, it is necessary to add potassium chloride to the regenerated fluoborate salt to provide the required electrolytic salt mixture.

The cathodes removed from the salt bath contain a deposit thereon of boron mixed with potassium fluoborate, potassium chloride, and in the non-continuous system, also, potassium fluoride. The cathode deposit may be subjected to water washing to separate the entrained bath salts, followed by a hydrochloric acid leach to remove minor amounts of acid soluble impurities from the boron. The salts in the resulting water wash solution may be recovered in accordance with the present invention by a number of different methods For example,

the salts may be recrystallized from the solution and thereafter, while in a molten state, reacted with a boron halide, e.g., boron trifluoride or boron trichloride, to remove any potassium fluoride. Preferably, however, the aqueous leach liquor itselfis treated with the boron halide until all of the potassium fluoride in the liquor has been transformed to the fluoborate, and then the salts are crystallized from the solution. After drying, the crystallized salts are again introduced into the electrolytic cell.

'As stated above, when the boron halide is boron trichloride, the reaction with potassium fluoride contained in the leach liquor will produce potassium chloride in addition to the fluoborate. When the boron halide is boron trifluoride, however, separate potassium chloride additions to the bath are necessary, along with the r e d leach quor salts, to pr vide or main a n Pr p electrolytic b h m t re .111 the ontinuous boron trichloride regeneration system, the cathode deposits contain substantially no potassium fluoride, and therefore, the salts recrystallized from the leach liquor consist substantially entirely of potassium chloride and and potassium fluoborate in the proper proportions and can be re-introduced into the electrolytic cell after only a drying step. In the same manner, when employing a continuous boron trifluoride regeneration system, the cathode deposits will contain substantially no potassium fluoride, and thus, the recovered salts may be returned to the cell after only a drying step. Since no potassium chloride is formed when boron trifluoride is employed, as stated above, it is necessary to add potassium chloride to the bath periodically.

The chlorine gas generated .in the electrolytic cell can be used, if desired, in the treatment of boron ores to form additional boron trichloride.

The advantages of the present invention are particularly important in the electrolysis of potassium fluoborate which has been produced in a form enriched in one or the other of the two boron isotopes, B and B According to published information (for example, Chemical Engineering, May 1957, pages 149-150), separation of the two isotopes of boron has been performed .by the US. Atomic Energy Commission by first converting ordinaryboron trifluoride into a volatile dimethyl ether-boron trifluoride complex. The volatile complex, containing but one atom of boron (either B or B in each molecule, is then subjected toga multistage distillation or fractionation'by which the material is separated into two fractions, one enriched with molecules containing the B isotope and the other enriched with molecules containing the B isotope. By repeating the distillation or fractionation in a multistage separation system, close to 100% separation can be achieved, but only at considerable processing cost in an elaborate physical plant involving a large capitalinvestment for its production capacity.

The final fractions from such an isotope separation process, respectively enriched with B or B to any desired degree, may then be individually converted to a correspondingly enriched potassium fluoborate for fused salt electrolysis by the process of the above-mentioned Cooper patent to produce elemental boron products that are also correspondingly enriched in one isotope or the other. This is presently being done bythe US. Atomic Energy Commission on a commercial scale to produce elemental boron enriched with the B isotope. However, the normal boron losses occurring in the electrolytic step represent far greater economic losses than when converting ordinary potassium fluoborate to elemental boron by that process. C

,By converting the isotope ;e nr iched dimethyl etherbQ TQn trifluoridecomplex into either boron trichloride or he 11 t ifl d an then us he ic l r d tor tr fluoride as the boron addition agent for'regenerating the fused saltelectrolytic bath in accordance with the present invention, the higheryields and greater efficiencies made possible by the present invention become of even greater economic importance.

Such conversion of the isotope enriched boron from the dimethyl ether-boron trifluoride complex into a correspondingly enriched boron trichloride may readily be accomplished, for example, by reacting the complex with aluminum trichloride to precipitate aluminum trifluoride and release substantially pure, gaseous, boron trichloride. Such conversion of the isotope enriched boron from the complex into a correspondingly enriched boron trifluoride may readily be accomplished, for example, by thermal decomposition.

Thus, the invention is particularly applicable and valuable when the boron compounds used as raw materials in the electrolytic and regeneration procedures have first been enriched at great expense with one or the other of the two boron isotopes.

The invention will be more fully described by the following illustrative examples, although it is to be understood that the invention is not limited to the details thereof:

Example I A conventional potassium fluoborate-potassium chloride bath (about 1 part fluoborate to 5 par ts chloride) was melted in a graphite crucible at a temperature of about 800 The bath was electrolyzedin the conventional manner by app-lying and maintaining a potential of about 5.5-8.0 volts, using the crucible as the anode and a pure iron cathode (1 0 x 15 cm.) suspended in the bath. The initial current was 400 amps. v From the time electrolysis was started, boron trichloride gas was continuously bubbled into .the .melt through a graphite tube inserted into the melt. Electrolysis was con tinued for a period of 4 hours and 50 minutes, during which time 3.6 kilograms of boron trichloride were introduced. The current remained steady at 400 amps. during the entire period. The cathode, having a boron deposit thereon together with entrained potassium chloride and potassium fluoborate, was :removed and a JXBW cathode inserted. The high purity boron (99+% boron) recovered after removing and washing the cathode deposit with water and acidweighed 312 grams. The electrolysis and the addition of boron trichloride were started again. The initial current was 600 amps. and remained steady for 5 /3 hours during which 5.2kil0- grams of boron trichloride were introduced.

At this point, the addition of boron trichloride was stopped. The current which had been 600.amps. dropped to amps. in a period of only 1 /2 hours. This is believed to have been due to the fact that boron trichloride had not been introduced at a high enough rate during theelectrolysis, with the result that the bath was partially exhausted and contained a substantial amount of potassium fluoride. At a current of 80 amps. the rate of boron deposit became so low that electrolysis was stopped. 590 grams of boron of a similarly high purity. were produced.

From the above, itcan be seen that, with electrolysis beingcontinued for :10 hours during the first and second electrolysis periods ,while continuously adding boron trichloride, the current carrying capacity of ,the'bath did not drop. ,However, when no boron trichloride was added, the current dropped severely in only 1% hours and necessitated termination of electrolysis.

Example 11 Apotassium fluoborate-potassium chloride electrolytic bath wasprepared by melting 1 8.2 kilograms of anhydrous potassium fluoride in a graphite crucible andadding 5.7 kilograms of boron trichloride thereto by bubb ins. h h 0 thwugh t e b t Il bath l e rose b u 8 c n im e uri the zasit tis 9 th he e? ih19id a I11 es l in a h-wat e. sl p l in the conventional manner, employing a graphite crut b e a the st re a a ra e iron s thqd swee ie? in the bath. Electrolysis was continued for 2% hours and produced 178 grams of high purity (99+%) boron.

The initial current was 800 amps. and the final current 600 amps. No observable diflerence was noted distinguishing this run from the conventional electrolytic method of the Cooper patent.

Example 111 Example IV The spent bath remaining from the electrolysis of Example III was regenerated by the addition of 3.75 kilograms of boron trichloride. The regenerated bath was then electrolyzed for 3 hours. The initial current was 580 amps. and the final current 500 amps. at about 5.5-8.0 volts. The yield of boron was 240 grams of a high purity product comparable to the products obtained in the previous examples.

Example V A conventional potassium fluoborate-potassium chloride bath containing these salts in the ratio of about 1 to 5, respectively, was electrolyzed for 2 hours and 40 minutes, during which period the current dropped from 800 amps. t 300 amps. Thereafter, 680 grams of boron trichloride were added continuously to the resulting bath over a period of 1 hour while continuing the current flow. Shortly after the flow of boron trichloride was started, the current rose quickly from 300 amps. to 600 amps. at the same voltage and remained at about 600 amps. for the hour during which the boron trichloride was added. Following completion of the addition of boron trichloride, the current slowly dropped to 400 amps. at the same voltage during an additional 1 /2 hours of electrolysis. 207 grams of high purity (99+%) boron were produced during the entire electrolytic period of more than 5 hours.

Example VI The spent bath of Example I remaining after the second electrolytic period and consisting substantially entirely of potassium fluoride was rejuvenated by the following procedure: To a 4.5 kilogram portion in a fused state was added 18.6 kilograms of potassium chloride. Boron trifiuoride gas was then introduced into the mixture to react with the potassium fluoride. About ten minutes after the o f on, trifiu ide gas had been started and after a portion of the potassium fluoride was converted to the fluoborate, electrolysis was started. The initial current was 600 amps. and remained at about this level for the entire 4 hour electrolytic period during which boron trifiuoride was continuously being added at a rate suflicient to react with the potassium fluoride but insuflicient to cause any significant loss of the trifiuoride into the atmosphere. The recovery of high-purity (99+%) boron was 235 grams.

The following examples illustrate the reaction of the boron halides with potassium fluoride in aqueous solution. This reaction can be employed either in the preparation of the initial electrolytic bath, or in the regenera' tion of potassium fluoride in the leach liquors recovered from the washing of the boron cathode deposits.

1000 ml. of water to produce a large quantity of a white precipitate. The total volume of liquid increased about 20% during the reaction period, and the temperature rose about 70 C., in spite of external water cooling. During the reaction, large, clear cubic crystals of potassium chloride could be seen forming in addition to the powdery potassium fluoborate precipitate.

The mixed potassium fluoborate-potassium chloride precipitate can be reused in an electrolytic bath by simple filtration and drying. However, to establish the yield of potassium fluoborate formed from the reaction of the potassium fluoride with the boron trichloride, the mix ture was diluted with an additional 1250 ml. of water. This dilution caused the potassium chloride crystals to redissolve while the potassium fluoborate remained as the precipitate. The remaining precipitate was filtered from the solution, dried, weighed and analyzed. The yield of potassium fluoborate was 345 grams compared to the calculated theoretical yield of 354 grams, giving a percentage yield of 97.5%. Analysis of the product revealed that the precipitate was iron-free, and contained 99.9+% potassium fluoborate.

Example VIII The procedure of Example VII was followed except that a solution containing 400 grams of potassium fluoride in 2000 ml. of water was treated with 260 grams of boron trifiuoride gas. Although somewhat more of the boron trifiuoride was lost, a product was obtained which analyzed 93.75% potassium fluoborate with a yield of 87.25%.

The invention as illustrated in the above description and examples not only provides a method for recovering the bath components but also a method for regenerating the bath itself, either batchwise or, more advantageously, in a continuous operation. This makes it possible for the electrolytic process itself to be carried out substantially continuously without any significant decrease in the current carrying capacity of the bath during electrolysis.

It will be apparent to one skilled in the art that the invention is not limited to the specific embodiments described in detail above, but that a number of modifications and variations can be made within the scope of the present invention as defined by the following claims.

What is claimed is:

1. In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in the bath, the steps of reacting in said bath a boron halide selected from the class consisting of boron trichloride and boron trifiuoride, with molten potassium fluoride in said bath to form molten potassium fluoborate, and electrolyzing the potassium fluoborate so produced while still in a molten state.

2. In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in the bath, the step of introducing into the bath a boron halide selected from the class consisting of boron trichloride and boron trifiuoride for reaction with molten potassium fluoride in said bath to form molten potassium fluoborate.

3. In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in the bath, the step of introducing boron trichloride into the bath for reaction with molten potassium fluoride in said bath to form molten potassium fluoborate and potassium chloride.

4. 'In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in said bath to form molten potassium boron trifiuoride into the bath for reaction with molten potassium fluoride in said bath to form molten potassium fluoborate.

5. j1n the processof making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to' deposit boron on the cathode and produce potassium fluoride inthe bath; the steps of substantially continuously introducing borontrichloride into the bath during electrolysis thereof for reaction with molten potassium fluoride formed in the bath during electrolysis to replenish the molten potassium'fluoborate and potassium chloride components of the bath without interruptingmolten potassium fluoborate in the bath, and adding potassium chloride to the bath aselectrolysis proceeds, whereby both the potassium fluoborate and potassium chloride components of the bath are replenished without interrupting electrolysis.

7. In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in the bath, the steps o f'introducing into the bath a boron halide selected from the class consisting of borontn'chloride and boron trifluoride for reaction with molten potassium fluoride in said bath to form molten potassium fluoborate; removing the cathode deposit from said bath; removing entrained bath salts from the boron in said cathode deposit; reacting-a boron halide selected from said class with potassium fluoride in said removed salts to produce potassium fluoborate; and returning the removed salts containing potassium fluoborate to said electrolytic bath. 7

8. In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride to deposit boron on the cathode and produce potassium fluoride in the bath, the steps of introducing into the bath a boron halide selected from the class consisting ofboron trichloride and boron trifluoride for reaction with-moltenpota'ssium fluoride in said bath to formmolten potassium fluoborate; removing the cathode deposit fromsaid bath;'removing entrained bath salts from the boron in said cathode deposit; reacting boron trichloride'withpotassium fluoride in said salts to produce potassium fluoborate and potassium chloride; and'returning to said electrolytic bath the potassium fluoborate and potassium chloride so produced.

9L In the process of making boron by electrolysis of a fused salt bath of potassium fluoborate and potassium chloride" to deposit boron on the cathode and" produce potassium fluoride in the bath, the steps of introducinginto the bath a boron" halide selected from the class consisting of; boron" trichloride'and' boron trifluor'ide ref action" with molten) potassium fluoride in said bath to form molten'potassium fluoborate; removing the cathode" deposit from said bath;:removingentrained bath salts from the boron in. said cathode deposit; reacting boron triflu'oride with potassium fluoride in said salts to' produce' potassium fluoborate; and returning the removed salts containing potassium fluoborate to said" electrolytic 10. In the process of making boron by electrolysis of a! removing the cathode deposit from saidbath; leaching the entrained bath salts. from the boron inasaid v atl'iode deposit; separating the dissolved salts from saidieac'h liquor, drying said salts; and returning the dried salts to said electrolytic bath.

References Cited in the file of this patent UNITED STATES PATENTS Cooper Oct. 23 19 5 1 Wainer rApr. 26, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No., 2,918,417 December 22, 1959 Hugh So Cooper et 6.1.,

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correct-ion and that the said Letters Patent should readas corrected below.

Column 6, line '72, for "said bath to form molten potassium" read the bath, the step of introducing Signed and sealed this 24th day of May 1960.,

(SEAL) Attest:

KARL H ,AXLINE Attesting Oflicer ROBERT C.. WATSON Commissioner of Patents 

1. IN THE PROCESS OF MAKING BORON BY ELECTROLYSIS OF A FUSED SALT BATH OF POTASSIUM FLUOBORATE AND POTASSIUM CHLORIDE TO DEPOSIT BORON ON THE CATHODE AND PRODUCE POTASSIUM FLUORIDE IN THE BATH, THE STEPS OF REACTING IN SAID BATH A BORON HALIDE SELECTED FROM THE CALSS CONSISTING OF BORON TRICHLORIDE AND BORON TRIFLUORIDE, WITH MOLTEN POTASSIUM FLUORINE IN SAID BATH TO FORM MOLTEN POTASSIUM FLUOBORATE, AND ELECTROLYZIND THE POTASSIUM FLUOBORATE SO PROUDCE WHILE STILL IN A MOLTEN STATE. 